1. Field of the Invention
The present invention relates to a tantalum compound suitable for forming, by the MOCVD method, a tantalum nitride film serving as a ground barrier in forming a copper film on semiconductor devices, a process for producing the same, and a method of forming a tantalum nitride film using the same.
2. Description of the Related Art
It is known that tantalum nitride is very effective as a ground barrier on semiconductor devices. For further thinning copper paths for electric current, further thinning of this barrier film is also required, and formation of a tantalum nitride film by the CVD method superior in mass-productivity and in the step coverage ability came to be necessary.
U.S. Pat. No. 5,668,054 discloses a CVD method using tantalum tertiary buthylimido tris(diethylamide) Ta(NtC4H9)(N(C2H5)2)3 (referred to hereinafter as Ta(NtBu)(NEt2)3 as the starting material, wherein a low-resistant tantalum nitride film is formed on a substrate at 450 to 650xc2x0 C. in a low-pressure reaction chamber of cold wall type at 0.02 Torr. The film at 600xc2x0 C. is preferable with resistance as low as 600 xcexcxcexa9xc2x7cm, and even upon treatment at a high temperature of 500xc2x0 C., can prevent copper diffusion and significantly lower leak current.
The starting material Ta(NtBu)(NEt2)3 is liquid at room temperature, and is supplied after gasification by heating at a source temperature of 40 to 50xc2x0 C.
In Appl. Phys. Lett., vol. 67(8)1128 (1995), the inventors of said prior art patent estimated that low-resistant cubic TaN could be formed because Taxe2x95x90NtBu in the starting compound, that is, the strong bond of Taxe2x95x90N was preserved and integrated in the cubic system. Before publication of said literature, said inventors disclosed in J. Mater. Sci. Lett., vol. 11, 96 (1992) that low-carbon cubic TaN was formed by CVD of Ta(NEt)(NEt2)3, but because a pure material of Ta(NEt)(NEt2)3 was hardly obtained, said inventors used Ta(NtBu)(NEt2)3 in place of Ta(NEt)(NEt2)3.
R. M. Fix, R. G. Gordon and D. M. Hoffman, in Chem. Mater., vol. 5, 614 (1993), have reported that cubic Ta3N5 film of high resistance ( greater than 106 xcexcxcexa9xc2x7cm) was formed in CVD using tantalum pentakis(dimethylamide) [Ta(NMe2)5] and ammonia.
U.S. Pat. No. 6,015,917 claims 10 groups of compounds having Taxe2x80x94N and/or Taxe2x95x90N bond as the starting material in CVD for forming a tantalum nitride film, as well as solutions thereof in hexane or toluene. In (viii), claim 1 describes, but does not specify, Ta(NR1)(NR2R3)3 wherein R1, R2 and R3 are independently from H, C1-C8 alkyl, aryl, C1-C8 perfluoroalkyl, and silicon-containing groups selected from the group consisting of silane, alkyl silane, perfluoroalkylsilyl, triaryl silane and alkylsilyl silane. This patent specification describes, in the Examples, only one compound of tantalum pentakis(dimethylamide) [Ta(NMeEt)5], and does not contain any description of the conditions for forming the film and the physical properties of the film.
JP-A 2000-204095 also discloses that a tantalum nitride film was formed from Ta(NMeEt)5 or a solution thereof in organic solvent and an ammonia gas at a substrate temperature of 450xc2x0 C. at a pressure of 10 Torr, but there is no description of the crystalline form of said film.
As described above, low-resistant cubic TaN has been obtained until now from only tantalum alkylimido tris(dialkylamide) having Taxe2x95x90N.
It is generally preferable that at the time of supply, the starting compound in CVD for mass-production has such properties that it is a pure material, has high vapor pressure, is thermally stable at the time of supply, and is liquid in the vicinity of room temperature or at least at the temperature of a source used. From these viewpoints, Ta(NtBu)(NEt2)3 in U.S. Pat. No. 5,668,054 is disadvantageous in that its vapor pressure is not high because of high molecular weight. According to measurement of vapor pressure by the present inventors, the vapor pressure thereof was 1 Torr/130xc2x0 C.
According to measurement of vapor pressure by the present inventors, a similar known compound, tantalum tertiary buthylimido tris(dimethylamide) [Ta(NtC4H9)(N(CH3)2)3] has a vapor pressure of 1 Torr/70xc2x0 C. which is higher than that of Ta(NtBu)(NEt2)3, but the problem of this compound is its melting point as high as 69xc2x0 C. For a usual liquid mass flow controller, the temperature which can be kept by heating for melting a starting compound is about 50xc2x0 C., and thus Ta(NtBu)(NMe2)3 with a melting point of 69xc2x0 C. is not suitable. The material desirably has a melting point in the vicinity of room temperature or about 40xc2x0 C. or less.
The present invention provides a novel compound having a higher vapor pressure than that of Ta(NtBu)(NEt2)3, a melting point of 40xc2x0 C. or less, and Taxe2x95x90N bond, as well as a process for producing the same. Further, the present invention provides a method of forming a cubic TaN film by using the same in the CVD method.
According to one aspect of the present invention, there is provided a novel compound, tantalum tertiary amylimido tris (dimethylamide).
According to another aspect of the present invention, there is provided a process for producing tantalum tertiary amylimido tris(dimethylamide), which comprises allowing 1 mole of tantalum pentachloride, 4 moles of lithium dimethylamide and 1 mole of lithium tertiary amylamide to react with one another in an organic solvent, then separating a byproduct lithium chloride by filtration, distilling the solvent away, and distilling the residue in vacuo.
According to still another aspect of the present invention, there is provided a solution of starting material for MOCVD, comprising at least 10 wt % organic solvent added to tantalum tertiary amylimido tris(dimethylamide).
According to still another aspect of the present invention, there is provided a method of forming a tantalum nitride film by the MOCVD method wherein tantalum tertiary amylimido tris (dimethylamide) is used as the starting material.